The present invention relates to a copier, printer, facsimile apparatus, multiplex machine or similar image forming apparatus. More particularly, the present invention relates to an electrophotographic image forming apparatus of the type repeating an image forming process, which includes charging, optical writing, development, image transfer and cleaning, with an image carrier to thereby sequentially form toner images on the image carrier, and sequentially transferring the resulting toner images to recording media, a cleaning unit for cleaning the surface of the image carrier, and a brush roller for the cleaning unit.
It is a common practice with an electrophotographic image forming apparatus to form, in a monochrome mode, a toner image on an image carrier and then transfer the toner image to a paper sheet, OHP (OverHead Projector) sheet or similar recording medium. After the image transfer, a cleaning unit removes toner left on the image carrier to thereby prepare the image carrier for the next image forming cycle.
In a color mode, monocolor images are sequentially formed on the image carrier while being sequentially transferred to an intermediate transfer body or another image carrier one above the other. The resulting color image completed on the intermediate transfer body is collectively transferred to a recording medium. After the image transfer, the surface of the photoconductive element and that of the intermediate image transfer body each are cleaned by a particular cleaning unit.
Each cleaning unit has customarily been implemented with a brush roller, a blade, a magnet brush or a bias roller. The brush roller has loop-like bristles or straight bristles implanted therein. The problem with loop-like bristles is that they press the image carrier, which is formed of OPC (Organic PhotoConductor) or similar resin, with a pressure several ten times to several hundred times higher than the pressure of straight bristles. The loop-like bristles therefore shave off the image carrier and cause a CTL (Carrier Transport Layer) included in the image carrier to wear, thereby reducing the life of the image carrier. Another problem is that such bristles cannot be densely implanted and therefore irregularly scrape off toner, resulting in stripe-like brush marks on the surface of the image carrier. The brush marks lower image density.
Not only toner but also ozone, NOx (nitrogen oxides) and other reactive gases produced by charging and image transfer deposit on the surface of the image carrier. Further, during image transfer, even talc, clay and paper fibers themselves deposit on the image carrier. Talk and clay are used to improve the quality of a paper surface. A cleaning member implemented by the loop brush can remove such deposits (contaminants), but a cleaning member implemented by any one of the straight brush, blade, magnet brush and bias roller cannot easily remove them. This is particularly true with an image carrier having fluorine-contained resin or wax on its surface and having a coefficient of friction of 0.2 or below, as measured by an Euler belt method.
As stated above, a conventional cleaning member with straight bristles cannot fully remove contaminants deposited on the image carrier although it can fully remove toner, as proved by experiments. Also, it was experimentally found that even bristles implanted in a higher density to increase the pressure of the entire brush, as taught in Japanese Patent Laid-Open Publication No. 9-288441 by way of example, failed to solve the above problem. Further, a greater amount of bite of the brush into the image carrier was not a solution to the problem either. The greater amount of bite caused the brush to deform (so-called creep) and thereby caused a torque to vary, resulting in irregular rotation and therefore noise and vibration.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 6-337598.
It is therefore an object of the present invention to improve the cleaning ability of a cleaning unit, which is included in an electrophotographic image forming apparatus, without reducing the life of an image carrier to thereby remove even contaminants, which would lower image quality.
It is another object of the present invention to more surely prevent image quality from being lowered in an electrophotographic image forming apparatus.
It is another object of the present invention to improve the cleaning ability by easily increasing the pressure of a brush.
It is another object of the present invention to allow a brush roller to smoothly rotate without excessively increasing a load to act on an image carrier or causing it to vary.
It is another object of the present invention to further extend the life of an image carrier and promote the easy migration of toner from the image carrier, thereby obviating defective images.
It is still another object of the present invention to provide the surface of an image carrier with a coefficient of friction of 0.3 or below with a simple configuration.
It is yet another object of the present invention to cope with both of a photoconductive element and an intermediate image transfer body, which are specific forms of an image carrier.
It is a further object of the present invention to achieve the above-described objects with a cleaning unit for an electrophotographic image forming apparatus and a brush roller included in the cleaning unit.
In accordance with the present invention, in an electrophotographic image forming apparatus including a brush roller for cleaning the surface of an image carrier, the brush roller has straight bristles implanted in a density xcfx81 (1/mm2) satisfying a relation:
xcfx81xe2x89xa730/xcfx80Ds 
where Ds denotes the diameter (mm) of a brush support included in the brush roller. The bristles each exert a mean pressure of 10xc3x9710xe2x88x925 (N) on the surface of the image carrier.